Method to produce graphene foam reinforced low temperature co-fired ceramic (LTCC) composites

ABSTRACT

A graphene foam ceramic composite (GrF-CC) comprises an open cell graphene foam (GrF) surrounded by and infiltrated with a sintered low temperature co-fired ceramic (LTCC) matrix. The GrF-CC can be prepared by infiltrating an open cell GrF with an LTCC slurry, removing the solvent from the slurry with solidification to a ceramic-GrF green body, and sintering the ceramic-GrF green body to form the GrF-CC. Sintering by spark plasma sintering (SPS) allows an LTCC GrF-CC that has a density of at least 90%.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/867,367, filed Jun. 27, 2019, which is hereby incorporated byreference herein in its entirety, including any figures, tables, anddrawings

BACKGROUND

Much attention has been given to 2D graphene for development of newcomposite materials with unprecedented properties and advancedapplications. Due to extraordinary mechanical, thermal and electricalproperties, graphene addition has been found to improve strength,toughness, stiffness and thermal-electrical conductivity of ceramicmaterials. Despite graphene's excellent intrinsic properties, there arenumerous practical challenges associated with engineeringgraphene-ceramic composites with desired microstructure and predictableproperties. Graphene flakes tend to agglomerate and form clustersarising from intermolecular 7C-7C interactions. These agglomerates actas stress concentrators within the microstructure that can initiatefailure upon mechanical loading. The non-homogeneous distribution canimpede electron and phonon conduction, limiting electrical and thermalproperties of the composites. To achieve homogeneity, various physicaland chemical dispersion techniques have been adapted to integrategraphene into the ceramic matrix including, alone or in combination,ball milling, sonication, centrifugation, surface modification, andfunctionalization. Unfortunately, these techniques are time-consuming,expensive, and/or involve the addition of chemicals/secondary particlesthat lead to undesirable impurities and can physically and chemicallydamage the graphene flakes. Additionally, due to the 2D morphology ofthe graphene flakes, the degree of alignment of the graphene flakes canlead to highly anisotropic and inconsistent properties. These processingand microstructure control challenges in the formation ofgraphene-ceramic composites constitute major bottlenecks in bulk scalemanufacturing and real-world application for this promising class ofnanocomposites.

To simplify the processing and improve the microstructure homogeneity,graphene with a free-standing 3D architecture as the filler material inthe composite can be used. Macroporous graphene foam (GrF) is apromising filler material for developing composites. GrF has anultra-low density (<5 mg/cm³), high surface area (˜850 m²/g), and largepore sizes (exceeding 500 μm). GrF consists of an interconnected networkof nodes and branches, providing seamless pathways for the transfer ofstresses, electrons, and phonons. The hierarchical structure of GrF hasbeen exploited by infiltration with polymer resin followed by curing tocreate a composite material with a defined and homogeneous distributionof the filler phase. By introduction of a pre-fabricated 3D foam,superior microstructure control and uniform properties result withoutthe need for complex dispersion techniques. As a result, this materialdisplays remarkable flexibility and resistance to failure, dampingcapability, and excellent electrical and thermal conductivities.GrF-reinforced composites are desirable in application for strainsensors, supercapacitors, electrochemical biosensors, biocompatiblescaffolds, electromagnetic shields, fuel cells, thermal interfaces,acoustic backers, vibration dampeners, and structural materials withenhanced failure resistance. Graphene foam's desirable mechanicalattributes have been exploited in metallic metamaterials with ultra-highstiffness, damage tolerance, and fatigue resistance.

GrF-ceramic composites (GrF-CCs) have been prepared for applications inLi-ion batteries, supercapacitors, biosensors, and high-performanceelectromagnetic interference shielding. These applications require highsurface area; hence, these composites are porous with a deposition ordecoration of ceramic on the GrF surface. No dense, non-porous 3DGrF-ceramic matrix composite for structural applications have beendemonstrated, even though a 3D GrF's interconnected network of branchescould allow unprecedented toughness and stiffness in a dense ceramic byfacilitating extensive stress transfer, crack deflection, and crackabsorption. Such toughness and stiffness has been reported for 2Dgraphene-based composites, presumably due to the rather localized anddiscrete nature of the reinforcement graphene phase. Hence, the 3Dcontinuous GrF holds the potential for superior ceramic composites withthe additional advantages of enhanced electrical and thermal properties.Typically, processing and densification of ceramics requires applicationof relatively high temperatures and pressures, which is a majorchallenge when the intent is to preserve the structural integrity of GrFduring ceramic densification.

BRIEF SUMMARY

Embodiments of the subject invention provide graphene foam ceramiccomposites (GrF-CCs) that include an open cell graphene foam (GrF)surrounded by and infiltrated with a sintered low temperature co-firedceramic (LTCC) matrix. The GrF constitutes an intra-connected scaffoldwithin the LTCC matrix. The GrF-CC can have a density of, for example,at least 90% or at least 95%. The GrF can be, for example, 0.1 wt %(weight percentage) to 1 wt % of the entire GrF-CC. The LTCC matrix cancomprise at least one of Al₂O₃ and Ca(Si,Al)₄O₈. The LTCC matrix can beat least 25% alumina ceramic (e.g., at least 25% by weight aluminaceramic and/or 25% by volume alumina ceramic).

Embodiments of the subject invention also provide methods of preparingan GrF-CC where an LTCC slurry comprising an LTCC powder, a dispersant,and a solvent is infiltrated into GrF to form a slurry-infiltrated GrF.Upon removal of the solvent, the slurry can solidify to a ceramic-GrFgreen body, which can subsequently be sintered to form the GrF-CC. TheLTCC slurry is effectively infiltrated when the ratio of the cellcross-section of the open cell GrF to the viscosity of the LTCC slurryis greater than 1 micrometer/centipoise (μm/cP). The GrF can have a cellcross-section in a range of from 100 μm to 700 μm. The LTCC powder cancomprise particles having a hydrodynamic diameter in a range of from 100nm to 20 μm, which can be included into the slurry at a loading of 20 wt% to 50 wt %. The dispersant can be included at, for example, 0.1 wt %to 2 wt %. The LTCC matrix can comprise at least 25% alumina. The LTCCpowder can be α-Al₂O₃ and/or Ca(Si,Al)₄O₈. The solvent can be removed byvaporization that is carried out by heating the solvent above itsboiling point (e.g., heating to 75° C. at atmospheric pressure). Thesolvent can be a single compound or a mixture of volatile compounds.Sintering can be carried out by spark plasma sintering (SPS) to achievevery high densities (e.g., at least 90% or even at least 95%). The SPScan be carried out at a temperature of, for example, 700° C. to 1500° C.and an applied pressure of, for example, 80 MPa to 200 MPa.

In an embodiment, the GrF-CC can be formed as a ceramic packagingmaterial. In an embodiment, the GrF-CC can be part of a medical implant.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic representation of a three steps fabricationmethod, according to an embodiment of the invention, to produce fullembedded closed cell GrF reinforced ceramic, according to an embodimentof the invention.

FIG. 2A shows a scanning electron microscope (SEM) image of an exemplaryfreestanding GrF used for fabricating a dense GrF-ceramic composite(GrF-CC), according to embodiments of the invention.

FIG. 2B shows a SEM image of an exemplary LTCC powder used forfabricating GrF-CC, according to embodiments of the invention.

FIG. 3 shows an SEM image displaying surviving GrF features after steptwo of the fabrication method, according to an embodiment of theinvention, where the insert magnified image shows a hollow GrF strut.

FIG. 4A shows an optical image of the LTCC/GrF GrF-CC cross-sectionafter SPS displaying the hexagonal geometry of the GrF cell.

FIG. 4B shows an optical image of the LTCC/GrF GrF-CC cross-sectionafter SPS displaying the unaltered branches and nodes of the GrF.

FIG. 5 shows force-displacement graphs of LTCC and LTCC/GrF GrF-CC,according to an embodiment of the invention, obtained using high loadindentation.

FIG. 6A shows a FIB-SEM image revealing crack deflection 90°out-of-plane for an exemplary LTCC/GrF GrF-CC, according to anembodiment of the invention.

FIG. 6B shows a FIB-SEM image revealing multiple crack deflection in anexemplary LTCC/GrF GrF-CC, according to an embodiment of the invention.

FIG. 7A shows a thermal image and a plot of temperature measurements atdifferent resolutions in pixels per inch of cylindrical LTCC/GrF GrF-CCsamples, according to an embodiment of the invention, and pure LTCCafter heating for 10 seconds.

FIG. 7B shows a thermal image and a plot of temperature measurements atdifferent resolutions in pixels per inch of cylindrical LTCC/GrF GrF-CCsamples, according to an embodiment of the invention, and pure LTCCafter heating for 20 seconds.

FIG. 7C shows a thermal image and a plot of temperature measurements atdifferent resolutions in pixels per inch of cylindrical LTCC/GrF GrF-CCsamples, according to an embodiment of the invention, and pure LTCCafter heating for 40 seconds.

FIG. 8 shows the dimensions and position of a GrF-CC, according to anembodiment of the invention, where a square GrF-CC portion is situatedcentrally within a cylindrical LTCC pellet.

DETAILED DESCRIPTION

Embodiments of the subject invention provide graphene foam ceramiccomposites (GrF-CCs) that include an open cell graphene foam (GrF)surrounded by and infiltrated with a sintered low temperature co-firedceramic (LTCC) matrix. The GrF constitutes an intra-connected scaffoldwithin the LTCC matrix. The GrF-CC can have a density of, for example,at least 90% or at least 95%. The GrF can be, for example, 0.1 wt % to 1wt % of the entire GrF-CC. The LTCC matrix can comprise at least one ofAl₂O₃ and Ca(Si,Al)₄O₈. The LTCC matrix can be at least 25% aluminaceramic.

Embodiments of the subject invention also provide methods of preparingan GrF-CC where an LTCC slurry comprising an LTCC powder, a dispersant,and a solvent is infiltrated into GrF to form a slurry-infiltrated GrF.Upon removal of the solvent, the slurry can solidify to a ceramic-GrFgreen body, which can subsequently be sintered to form the GrF-CC. TheLTCC slurry is effectively infiltrated when the ratio of the cellcross-section of the open cell GrF to the viscosity of the LTCC slurryis greater than 1 micrometer/centipoise (μm/cP). The GrF can have a cellcross-section in a range of from 100 μm to 700 μm. The LTCC powder cancomprise particles having a hydrodynamic diameter in a range of from 100nm to 20 μm, which can be included into the slurry at a loading of 20 wt% to 50 wt %. The dispersant can be included at, for example, 0.1 wt %to 2 wt %. The LTCC matrix can comprise at least 25% alumina. The LTCCpowder can be α-Al₂O₃ and/or Ca(Si,Al)₄O₈. The solvent can be removed byvaporization that is carried out by heating the solvent above itsboiling point (e.g., heating to 75° C. at atmospheric pressure). Thesolvent can be a single compound or a mixture of volatile compounds.Sintering can be carried out by spark plasma sintering (SPS) to achievevery high densities (e.g., at least 90% or even at least 95%). The SPScan be carried out at a temperature of, for example, 700° C. to 1500° C.and an applied pressure of, for example, 80 MPa to 200 MPa.

The sintering can also be done with no applied pressure at an elevatedtemperature (e.g., in a range of from 700° C. to 1500° C., such as at850° C.) for a period of time (e.g., 20 minutes) in an inert atmosphere(e.g., argon) to ensure complete solidification of the LTCC slurry inthe porous 3D graphene structure.

Embodiments of the subject invention provide dense 3D graphene foamceramic composites (GrF-CCs) and a three-step method to fabricate suchdense 3D GrF-CCs. The method involves: infiltration of ceramic slurryinto a GrF; drying and solidification of the slurry; andpressure-assisted sintering of the ceramic-filled GrF. One or more GrFsresides within at least a portion of a continuous ceramic matrix. Uponcompletion of the infiltration and solidification of the ceramic, aceramic-GrF green body forms the structural backbone of the porous GrFdue to a solidified ceramic phase. This ceramic-GrF green body issubsequently sintered without compromising the 3D interconnectedarchitecture of GrF. The ceramic is, for example, an alumina and silica,α-Al₂O₃—Ca(Si, Al)₄O₈, low temperature co-fired ceramic (LTCC) that israpidly densified by spark plasma sintering (SPS) to achieve an intimateceramic/graphene interface within the composite. Low temperatureco-fired ceramics that can be used are compositions where the ratio ofAl₂O₃ ceramic to a glass is, for example, but not limited to, about 1:3.Common glass powder materials within the LTCC includecalicia-magnesia-alumina, silicate glass, and Borosilicate glass. Theglass powder adjusts the thermal expansion coefficient of the ceramicand reduces the sintering temperature of the pure Al₂O₃, which is about1400° C. The type and proportion of glass adjust the material'sdielectric constant. Ceramic and glass powders for the LTCC aregenerally ball grinded after mixing to achieve a uniform distribution ofthe desired sized particle, although any form of high shear mixing canbe employed.

LTCC have widespread applications in packaging of electronic systems inautomotive, military, space, medical and industrial installations. LTCCsare commonly integrated into Microelectromechanical systems (MEMS).Superior mechanical properties and thermal transport are desirable inLTCCs to have structural stability, thermal management, and productreliability. 3D GrF allows simultaneous enhancement of mechanical andthermal properties of LTCC at a low loading in wt %. GrF content of lessthan 1 wt % can enhance the mechanical, thermal, and electricalproperties of the GrF-CC. GrF content of the GrF-CC can be, for example,1, 0.9, 0.8, 0.7, 0.4, 0.5, 0.6, 0.3, 0.2, 0.1, or any value between 0.1and 1 inclusive (all numerical values are wt %). The SPS techniqueallows fabrication of GrF-CC that preserves the GrF's reticulated cellstructure. The highly reticulated 3D GrF structure is retained underhigh pressure and temperature conditions required to consolidate a denseceramic by the method according to an embodiment of the invention. Thiscontrasts with the normal handling of freestanding GrF, which isparticularly delicate, and high pressure can easily destroy itsinterconnected and continuous structure.

The methods, according to embodiments of the subject invention, allowmultifunctional ceramic composite materials with microstructuralhomogeneity, and superior mechanical and transport properties. Athree-step method of an embodiment is schematically represented in FIG.1.

The first step of the method is the infiltration of GrF by a lowviscosity LTCC slurry, as shown in FIG. 1, to form a slurry infiltratedGrF. For example, the LTCC ceramic slurry can include 32 wt % of solidsalong with 0.57 wt % of fish oil dispersant in a terpineol solvent, asused during reduction to practice of a LTCC GrF-CC, in an embodiment.The solid loading of the LTCC ceramic slurry can be, for example, 20 wt% to 50 wt % and the dispersant can be, for example, 0.1 wt % to 2 wt %.Other dispersants and solvents can be employed and other components canbe included in the LTCC comprising slurry. Other compositions of theslurry are possible as long as the ceramic particles are relativelysmaller than the pore size of the GrF, as illustrated in FIG. 2A, wherethe pore size and shape of a free-standing GrF can be filled with theLTCC powder, as illustrated in FIG. 2B. In an embodiment, the averageparticle size of the LTCC is <3 and is readily packed within theapproximately 540 μm pores of the GrF. The GrF can have open cell poresthat are, for example, 100 μm to 700 μm in cross-section and the LTCCcan have particles with a hydrodynamic diameter of, for example, 100 nmto 20 μm. By including additional components into the LTCC comprisingslurry, where the additional components are of similar hydrodynamicdiameter to that of the ceramic, the electrical conductivity, thermalconductivity, or mechanical strength can be independently augmented,particularly when the additional components are employed at sufficientquantity to achieve a percolation threshold of the additional componentparticles within the LTCC matrix. For example, by the inclusion ofdiamond particles into the slurry, the thermal conductivity of theGrF-CC can be improved without significant augmentation of theelectrical conductivity of the GrF-CC. For example, by inclusion ofgraphene flakes into the slurry, the electrical conductivity of theGrF-CC can be further enhanced. The irregular shape of the LTCC powderparticles assist in efficient powder packing during SPS. Slurrycompositions are formulated based on an infiltration factor (I_(f)),which is the ratio of the cell size of the GrF to the viscosity of theinfiltrate slurry. For efficient infiltration, the I_(f) is greater thanor about equal to 1 μm/cP. For example, terpineol, having a viscosity of36.5 cP at RT, is used to form an LTCC slurry by 1 hour of ball millingthe ceramic powder to yield a slurry viscosity of 80 cP at RT. For a GrFcell size of 540 μm, the I_(f) value is 6.75 μm/cP. At this I_(f) value,the slurry infiltrates the GrF efficiently to convert the open cell GrFto a completely closed cell GrF, as shown in Step 1 of the schematic inFIG. 1. For example, slurry viscosities can be 50 cP to 400 cP.

The second step of the method, according to an embodiment, is theevaporation of the solvent and solidification of the infiltrated LTCCslurry. This is achieved by a two-step heat treatment process.Initially, the slurry infiltrated GrF is heated to a first temperaturefor a first period of time, for example, but not limited to, 75° C. for30 minutes, to evaporate solvent. Subsequent heating to a secondtemperature, for example, but not limited to, 850° C. for 20 minutes inan argon atmosphere completes the solidification. Typically, evaporationis at a temperature well above the boiling point of the solvent, whichcan be a mixture of solvents, and carried out for a sufficient time toremove the majority, for example, but not limited to about 95% of thesolvent. The densified particulate LTCC encapsulates and fills the poresof the GrF, which act as a protective barrier to impart load-bearingfunctionality to the GrF-ceramic green body. The GrF's reticulatedstructure is retained under application of an external pressure by theconfined LTCC in the GrF cells. This is advantageous for fabrication ofelectronic packaging for medical implants, according to an embodiment ofthe invention, where an electronic circuit is designed and is embeddedon the GrF in Step 1 of the method, followed by solidification of theLTCC around the electronic circuitry to protect the circuit elementsfrom external pressure to form a robust implant. Step 1 and 2 arecrucial in the formation of the GrF-CC device.

Preservation of reticulated GrF foam structure during fabrication of theGrF-CC is important for achievement of the desired properties, accordingto an embodiment of the invention. FIG. 3 shows an SEM image of asurface of the green body LTCC/GrF after step two of the fabricationmethod illustrated in FIG. 1. GrF features, like nodes and branches,remain intact, as shown in FIG. 3. In step 1, infiltration occurswithout pressure and results from the gravity assisted infill of theLTCC slurry. The surface shown in FIG. 3 appears relatively dense with agood mechanical bonding between the graphene and LTCC. The de-bondedregion, shown in the inset of FIG. 3, is the hollow strut of the GrFthat resulted from etching of the Ni scaffold during a GrF fabricationprocess by CVD. The hollow struts are about 30-50 μm in diameter. Eventhough the average particle size of LTCC powder is <3 μm, infiltrationfails to fill the hollow struts, and it appears that the Laplacepressure does not allow resin infiltration into the more narrow hollowstruts due to capillary action, where the Laplace pressure act only onthe liquid portion, and has little effect on the micron-sized solidceramic particle portion. The inset shown in FIG. 3, infers intimatebonding of GrF's branches with the sintered LTCC matrix. The hollowinterfacial profile can be eliminated during consolidation via SPS.

Step 3 of the method, according to an embodiment, is densification ofLTCC/GrF green body by SPS. SPS is a technique that has been developedfor rapid densification of ceramic materials, hard-metals, cermets,Al-based alloys, and other metallic powders. The LTCC/GrF green body issandwiched between LTCC powder beds in a graphite die. The die issubjected to the sintering conditions to obtain a dense ceramicreinforced with GrF as a LTCC GrF-CC, according to an embodiment of theinvention, with the GrF reticulated structure intact, as shown inFIG. 1. The GrF-CC can have a density of at least 90%, for example, 90,91, 92, 93, 94, 95, 96, 97, 98 or 99%. From the optical images shown inFIGS. 4A and 4B, it is evident that the reticulated structure of the GrFremains intact after the GrF is subjected to high temperature andpressure during SPS. The silica in the LTCC composition allows effectivedensification of the ceramic at an SPS temperature as low as 700° C. TheGrF in the ceramic amplifies the thermal and electrical conductivity inthe dense GrF-CC. An exemplary sintered LTCC/GrF GrF-CC is 97.2% dense(i.e., has a density of 97.2%), whereas a comparative SPS sintered LTCCis 98.5% dense. The reduction in density is consistent with the presenceof hollow struts of the GrF as shown in FIG. 3.

The infiltrated LTCC slurry provides sufficient support to allow the GrFto withstand the pressure of 100 MPa applied during SPS. The hexagonalprofile of the embedded GrF, FIG. 4A, along with its branches and nodes,FIG. 4B, are visible in the SPS consolidated GrF-ceramic composite.Moreover, the hollow branches, as shown in the inset in FIG. 3, collapseinto layers of graphene after sintering. Hence, the collapsed branches,in addition to cells filled with LTCC, are desirable for enhanced loadbearing and stress transfer functionality in the material.

The GrF reinforcement on the mechanical properties of the LTCC/GrFGrF-CC is evident from an evaluation using a high load, 100 N,instrumented indentation technique. Indentation test was performed onthe polished cross-section of pure LTCC and GrF-LTCC composite samples.FIG. 5 shows load-displacement graphs portraying the behavior of theGrF-CC and LTCC samples under uniaxial compressive load in adisplacement control mode. For a similar displacement of 0.25 mm, theexemplary LTCC/GrF GrF-CC can withstand about 4 times the load of theexemplary LTCC. The displacement recovery of the exemplary LTCC/GrFGrF-CC material is ˜23% compared to the exemplary pure LTCC. GrF isknown for its super-elastic metamaterial properties. This creates a sitefor efficient energy dissipation thus absorbing the deformation energycaused by loading. The area under the force-displacement curve in FIG. 5represents the energy absorbed by the material upon deformation andfracture. The fracture energy for the exemplary LTCC/GrF GrF-CC is 7.54mJ, as compared to 1.3 mJ for the exemplary LTCC, which is a ˜480%improvement in the fracture energy of the LTCC by inclusion of a 0.18 wt% GrF reinforcement. Due to the intrinsic brittle nature of the LTCC,energy dissipated during high load indentation is released in the formof cracks nucleated from the indent. The measured crack length was1031.2 μm for the exemplary LTCC sample as compared to a much shortercrack length of 215.5 μm for the exemplary LTCC/GrF GrF-CC. To establishthe dominant fracture mechanism in LTCC/GrF GrF-CCs, the crack generatedby indentation on an exemplary composite was investigated by focused ionbeam-scanning electron microscopy (FIB-SEM) as shown in FIGS. 6A and 6B.The inset in FIG. 6A shows a crack emanating from a region below theindent. The crack does not propagate as a continuous line, instead thecrack manifest as a discontinuous line. FIG. 6A is the FIB-SEM of thesub-surface showing the crack which deflected 90° out-of-plane. Thissuggests that the GrF alters the behavior of crack propagation and crackdeflection can be the dominant fracture mechanism as multiple crackdeflection is indicated in FIG. 6B.

To establish the influence of GrF on the electrical property of LTCC, afour-point probe DC electrical characterization at room temperature andpressure was conducted. Pure LTCC (Dupont 951) is an electricallyinsulating material with a dielectric constant of 7.8. Hence it iswidely used in the fabrication of micro-resistors and micro-capacitors.Due to the high electrical resistance above the range of a DMM used, nospecific resistance value was obtained for a pure LTCC sample. For theexemplary LTCC/GrF GrF-CC, electrical conductivity is dominated by theGrF's intrinsic properties and the material recorded a total electricalconductivity of 165 S/m. The high electrical conductivity in 3D GrF, ofabout 10⁶ S/m, stems from the adjacent graphene sheets being bonded byvan der Waals force resulting in contact conductance. The electricalconductivity of GrF is highly influenced by the bending stress at thenode and adhesion energy between individual Gr sheets at acharacteristic length scale. The length and width of individual Grsheets forming the holistic GrF also contribute to its electron andphonon transfer phenomenon. The LTCC infiltrate may exert compression onthe GrF within the GrF-CC and increase the contact area betweenindividual Gr sheets to increase the electrical property by contactconductance phenomenon. The induced electrical conductivity in theLTCC/GrF is consistent with a reticulated structure of the GrF that isintact after SPS. The conductivity can be modified by the GrF content inthe GrF-CC.

The thermal property of GrF-CCs can be shown by thermal analysis usingan infrared camera. An exemplary LTCC/GrF GrF-CC and exemplary LTCCsample were placed on an isothermal hotplate maintained at 100° C. Thechange in sample temperature with time was monitored for both samples.FIGS. 7A-7C shows thermal images of the samples at different times atdifferent sample temperature. As indicated in FIGS. 7A-7C, the LTCC/GrFGrF-CC conducts heat at a higher rate than pure LTCC sample, and theLTCC/GrF GrF-CC conducts heat more uniformly across the bulk as observedby a more uniform coloration of the test samples. At a very low GrF wt%, as in the exemplary 0.18 wt %, the GrF-CC exhibits not only improvedmechanical properties but also displays enhanced electrical and thermalconductivity due to electron and phonon transfer, respectively. Theimprovement in thermal conductivity is in the range of 1° C./s, evenwhen a small portion of the sample, as in the exemplary 20 mm³ of GrF-CCis present within the much larger, about 638 mm³, of the cylindricalsample, as indicated in FIG. 8.

By controlling the wt % of GrF in the GrF-CC, one can fine tune thethermal and electrical properties of the material along with mechanicalenhancement. The highly reticulated 3D architecture of the GrF in theGrF-CC can be used as an improved ceramic packaging material. The closedcell, fully embedded GrF-CC can be used as components for medicalimplants or sensors. The processing technology enables in thefabrication of advanced engineering materials with multifunctionalproperties, displaying excellent mechanical, electrical and thermalproperties, for use in aggressive environments, such as, but not limitedto, satellite communication systems, automobile control systems, andaerospace control systems.

Methods and Materials

Free-standing GrF was received from Graphene Supermarket (Calverton,N.Y., USA). The LTCC powder was obtained from Dupont (Wilmington, Del.,USA) commercially known as Dupont 951. Though the exact composition ofthe powder is proprietary, the major phase structure is composed ofα-Al₂O₃ and Ca(Si,Al)₄O₈. The viscosity of the ceramic slurry wasmeasured by Brooksfield DV-II viscometer. SPS was performed at 700° C.for 15 minutes at 50° C./min at an applied pressure of 100 MPa.

Field-emission scanning electron microscope (FE-SEM JEOL JSM-6330F, JEOLLtd. Tokyo, Japan) was employed to inspect the top and fracture surfacesof LTCC and LTCC/GrF GrF-CC samples. Optical Microscope (Versamet 3,Buehler, Lake Bluff, Ill., USA) was used to ensure the retention of theGrF's 3D architecture after each processing step. Mechanical propertieswere analyzed on sintered pellets. A 100 N instrumented indentation wasperformed by a linear, screw driven micro-load frame (SEM Tester 1000,MTI Instruments Inc. USA.) with an attached Vickers tip. Indentationtests were carried out on a polished surface with a maximum displacementof 0.25 mm. A Multi-Beam Focused Ion Beam (FIB, JEOL-JIB 4500, JEOL Ltd.Tokyo, Japan) was employed to mill the cracked surface for observing thesub-surface fracture mechanism in the LTCC/GrF composite material. Theelectrical conductivity of the LTCC/GrF GrF-CC was evaluated using a DCfour-point-probe method with a Keithly 2401 digital multimeter at roomtemperature and pressure. A T450 sc thermal imaging camera from FLIR wasemployed for thermal studies.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

What is claimed is:
 1. A method of preparing a graphene foam ceramiccomposite (GrF-CC), the GrF-CC comprising an open cell graphene foam(GrF) surrounded by and infiltrated with a sintered low temperatureco-fired ceramic (LTCC) matrix, the GrF comprising an intra-connectedscaffold within at least a portion of the LTCC matrix, and the GrF-CCbeing at least 90% dense, the method comprising: providing the open cellGrF; providing an LTCC slurry comprising an LTCC powder, a dispersant,and a solvent; infiltrating the GrF with the LTCC slurry to form aslurry infiltrated GrF; removing the solvent from the slurry infiltratedGrF with solidification of a ceramic-GrF green body; and sintering theceramic-GrF green body to form the GrF-CC.
 2. The method according toclaim 1, the ratio of a cell cross-section of the open cell GrF to aviscosity of the LTCC slurry being greater than 1 μm/cP.
 3. The methodaccording to claim 2, the cell cross-section of the open cell GrF beingin a range of from 100 μm to 700 μm, and the LTCC powder comprisingparticles with a hydrodynamic diameter in a range of from 100 nm to 20μm.
 4. The method according to claim 1, the LTCC powder being 20 wt % to50 wt % of the LTCC slurry.
 5. The method according to claim 1, thedispersant being 0.1 wt % to 2 wt % of the LTCC slurry.
 6. The methodaccording to claim 1, the LTCC powder comprising at least 25% alumina.7. The method according to claim 1, the LTCC powder comprising α-Al₂O₃and Ca(Si,Al)₄O₈.
 8. The method according to claim 1, the removing ofthe solvent comprising vaporizing the solvent.
 9. The method accordingto claim 1, the sintering being performed with no applied pressure in aninert atmosphere and at a temperature in a range of from 700° C. to1500° C.
 10. The method according to claim 1, the solvent comprising oneor more volatile compounds.
 11. The method according to claim 1, thedispersant being fish oil and the solvent comprising at least oneterpineol, the sintering being spark plasma sintering (SPS), the SPSbeing performed at a temperature in a range of from 700° C. to 1500° C.,and the SPS being performed at an applied pressure in a range of from 80MPa to 200 MPa.
 12. The method according to claim 11, the ratio of acell cross-section of the open cell GrF to a viscosity of the LTCCslurry being greater than 1 μm/cP, the cell cross-section of the opencell GrF being in a range of from 100 μm to 700 μm, the LTCC powdercomprising particles with a hydrodynamic diameter in a range of from 100nm to 20 μm, the LTCC powder being 20 wt % to 50 wt % of the LTCCslurry, the dispersant being 0.1 wt % to 2 wt % of the LTCC slurry, theLTCC powder comprising at least 25% alumina, the LTCC powder comprisingα-Al₂O₃ and Ca(Si,Al)₄O₈, the removing of the solvent comprisingvaporizing the solvent, the removing of the solvent being carried out atambient pressure, the removing of the solvent being carried out at atemperature at or above the boiling point of the solvent, and thesolvent comprising one or more volatile compounds.
 13. A method ofpreparing a graphene foam ceramic composite (GrF-CC), the GrF-CCcomprising an open cell graphene foam (GrF) surrounded by andinfiltrated with a sintered low temperature co-fired ceramic (LTCC)matrix, the GrF comprising an intra-connected scaffold within at least aportion of the LTCC matrix, and the GrF-CC being at least 90% dense, themethod comprising: providing the open cell GrF; providing an LTCC slurrycomprising an LTCC powder, a dispersant, and a solvent; infiltrating theGrF with the LTCC slurry to form a slurry infiltrated GrF; removing thesolvent from the slurry infiltrated GrF with solidification of aceramic-GrF green body; and sintering the ceramic-GrF green body to formthe GrF-CC, the ratio of a cell cross-section of the open cell GrF to aviscosity of the LTCC slurry being greater than 1 μm/cP, the cellcross-section of the open cell GrF being in a range of from 100 μm to700 μm, the LTCC powder comprising particles with a hydrodynamicdiameter in a range of from 100 nm to 20 μm, the LTCC powder being 20 wt% to 50 wt % of the LTCC slurry, the dispersant being 0.1 wt % to 2 wt %of the LTCC slurry, the LTCC powder comprising at least 25% alumina, theLTCC powder comprising α-Al₂O₃ and Ca(Si,Al)₄O₈, the removing of thesolvent comprising vaporizing the solvent, the solvent comprising one ormore volatile compounds, the dispersant being fish oil and the solventcomprising at least one terpineol, and the sintering being performedwith no applied pressure in an inert atmosphere and at a temperature ina range of from 700° C. to 1500° C.